Wavelength division multiplexing (WDM) in passive optical networks (PON) dramatically increases the fibre link capacity compared with other fibre access techniques, especially if the same optical carrier is used for both downstream and the upstream signals propagating in the same fibre. WDM PON systems that reuse the same wavelength for both downlink and uplink transmission allow the system capacity and number of users to be doubled as compared to a WDM PON where a pair of wavelengths are used for each bidirectional channel. However, reusing the same wavelength for upstream transmission requires the downstream data modulation to be removed before applying the upstream data. This leads to an additional power penalty on the upstream signal, caused by residual downstream modulation and optical reflections along the optical link.
One solution uses a reflective semiconductor optical amplifier (RSOA) as a colourless transmitter at the optical network unit (ONU). Part of the downstream signal is tapped and sent to the RSOA input. If the power of the tapped signal is sufficiently high (typically greater than −10 dBm) it saturates the RSOA, cleaning the downstream modulation so that the upstream data can be applied to the RSOA electrical input. This solution faces the problem that the high input optical power needed to saturate the RSOA limits the achievable span budget in practical systems, giving the PON a typical reach of less than 5 km.
In a second solution, the downstream traffic is FSK (Frequency Shift Key) modulated by using a directly modulated laser diode, properly biased in order to have small residual amplitude modulation and a sharp chirp response, to maximize the frequency shift caused by any driving current variation. At the ONU, the received signal is split in two parts: one part is sent to an optical filter in front of the downstream receiver while the other part is used as optical carrier by the upstream data, and is remodulated with upstream data by an RSOA. The function of the filter is to convert the FSK modulation format into an ASK (Amplitude Shift Keying) format. This is achieved by positioning the optical carrier frequency on one of slope edges of the optical filter. A similar scheme exploits DPSK (Differential Phase Shift Keying) modulation instead of FSK: this requires a phase modulator at the downstream transmitter and an interferometer in place of the optical filter for ASK conversion. The cost of the filter or interferometer in these arrangements is a disadvantage, and additional elements are required to provide thermal control of the filter or dedicated circuitry to track any drift of the downstream optical carrier.
A further solution, reported by Presi et al, “A 80 km reach fully passive WDM-PON based on reflective ONUs”, Optics Express, vol. 16, no. 23, pp 19043-19048, 10 Nov. 2008, uses RZ (Return to Zero) and IRZ (Inverse Return to Zero) modulation formats for upstream and downstream transmission, respectively. An RSOA at the ONU remodulates and amplifies a seed portion of a received downstream IRZ signal to generate an RZ upstream data signal. The RSOA is operated outside of the saturation regime, which, together with using the IRZ/RZ line coding pair, increases the power budget and thus the maximum achievable reach of the PON.